Non-contacting temperature sensing device

ABSTRACT

The device according to the invention provides a non-contacting temperature sensing device incorporating micro-bolometric detectors as the suitable infrared sensors for automotive applications. A first and second infrared sensors each include an active infrared sensing element and a temperature drift compensating element. A current bias is applied to the active infrared sensing element as well as to the temperature drift compensating element, which is identical in structure with the active infrared sensing element, and the voltage outputs of these two elements pass through a differential amplifier. The fluctuation in the substrate temperature or the ambient temperature affects the active sensing element and the compensating element in the same way, thus it is cancelled out. Instead of using one spectral band of the infrared radiation, as in the prior art, two spectral bands are used resulting in a first and second signal generated by the first and second infrared sensors. A ratio of the first and second signals is obtained. The ratio of the signals is emissivity independent, so that the device of the present invention provides a more accurate measurement of temperature. The need to compensate for window contamination is also eliminated by this two band approach. The filtering elements for the two bands can be multi-layer thin film filters either coated on flat windows or on diffractive micro-lenses. The use of diffractive micro-lenses further reduces the size of the device, and eliminates the need for a separate optical lens.

FIELD OF THE INVENTION

[0001] The present invention relates to a device for the non-contactingtemperature measurement of a surface, for example, the surface of aroadway, from a moving vehicle, using an emissivity independent infraredradiation detection method and infrared detectors, although theinvention is not limited in scope to automotive applications.

DESCRIPTION OF THE PRIOR ART

[0002] One important aspect of automobile related inventions is toimprove the safety features. It has been found desirable to be able toremotely measure the road surface temperature from a moving vehicle inan accurate way, either for predicting the likelihood of icing formationor for warning of overheated surface. Earlier road condition monitoringsystems used ordinary thermistors to measure the ambient temperatureclose to road surface as an approximation for the actual surfacetemperature. Essentially, these systems include a temperature sensingmeans for measuring the temperature and means for providing a warning ifthe measured temperature falls within a range indicative of ice-formingconditions.

[0003] Infrared sensors have long been used for remote measurement oftemperature of a surface based on the fact that when the infraredradiation associated with the temperature of the object impinges on andheats an infrared sensor, it induces a change in the sensor outputproportional to the infrared radiation the sensor receives. However,most of such applications are for relatively high temperaturemeasurements (>100° C.). In the case of surfaces with temperatures nearor below room temperature, for example, the road surface temperature inwinter, the infrared radiation therefrom impinging on the infraredsensor is very weak. Therefore, not all types of infrared sensors aresuitable for such applications.

[0004] U.S. Pat. No. 5,416,476 by Rendon proposes using infrareddetectors as the temperature sensing means in order to detectpotentially icy conditions on roads. This patent describes a system, andmethod for detecting potentially icy conditions on roads with aninfrared detector mounted externally of a vehicle and aimed at a roadsurface. The detector is arranged to read only the infrared temperaturewavelength emissions associated with concrete and asphalt to eliminateerroneous readings inadvertently received through infrared emissions ofother objects located in the vicinity of the detector. The detector isconnected to a processing unit which translates the electrical signalsfrom the detector into a temperature readout display. However, thispatent does not state or suggest which type of infrared detectors aresuitable candidates.

[0005] U.S. Pat. No. 5,796,344 by Mann, et al. further elaborated thesignal processing aspect of such systems with consideration forcompensation of window contamination from road dust and spray, since theinfrared detectors are usually housed in a casing provided with awindow. Due to various factors, the window tends to become contaminatedwith dust, water, spray, etc., while the vehicle is in motion.

[0006] These two patents have considered neither the impact of theemissivity variation on the accuracy of the temperature measurement, northe compensation for the detector substrate and background temperaturedrift due to the normal operation of a vehicle.

[0007] The infrared detector types mentioned by Mann et al. arethermopile or pyroelectric type infrared sensors. Thermopile detectorsare known to have long thermal time constant, thus can not respond tofast changes in temperature. Pyroelectric type detectors areintrinsically sensitive to mechanical vibration and shocks, thus are notsuitable in a vibrating environment such as on-board a moving vehicle.Pyroelectric type detectors also need a chopper to modulate the incidentradiation, complicating the system.

[0008] The amount of infrared radiation received by the detector dependson the temperature and the emissivity of the radiating surface. Earlierroad temperature measuring devices using infrared detection, representedby Rendon and Mann, et al., detect the infrared radiation within onewavelength band and calculate the temperature based on the assumptionthat the emissivity of the road surface is a constant value. However,this is a very rough approximation. Different road constructionmaterials can have different emissivity. Under different weatherconditions, the same material can also have a different emissivity.Therefore, the temperature interpretation based on constant emissivityassumption is not always valid.

[0009] Micro-bolometric detectors have been developed only recentlythanks to advanced micro-machining technology. A micro-bolometer is asuspended structure, either as a raised platform over a substratethrough surface micro-machining technique, or a flat platform over acavity in the substrate through bulk micro-machining technique. FIG. 1,identified as prior art, shows the structure of a typicalmicro-bolometer 40, consisting of a suspended infrared sensing platform43 supported by thin, long beams 42, formed on a semiconductor substrate41. The platform 43 is formed with infrared sensitive materials, forexample VO, Amorphous Silicon, Ti nitride, etc. sandwiched betweeninsulating dielectric layers. The resistance of the infrared sensitivematerial decreases as the temperature of the sensor increases due to theinfrared radiation. The thin, long supporting beams greatly reduce thethermal diffusion into the substrate, and thus improve the thermalisolation. Therefore, the thermal sensitivity of a micro-bolometer ishigh compared with other uncooled thermal infrared detectors. Sincemicro-bolometers can be integrated with on-chip electronic circuits, theinfrared sensing module can be made on a single chip at a low cost.Micro-bolometers are not as susceptible to vibration or shocks as thepyroelectric detectors are.

[0010] In the field of systems for detecting potentially icy conditionson roads, reference may be made to the following United States patents:U.S. Patent Issue Date Inventor(s) 3,596,264 Jul.27, 1971 Ciemochowski3,891,979 Jun.24, 1975 Braun 4,222,044 Sep. 9, 1980 Boschung 4,492,952Jan. 8, 1985 Miller 5,416,476 May 16, 1995 Rendon 5,796,344 Aug.18, 1998Mann, et al

SUMMARY OF THE INVENTION

[0011] It is an object of the invention to provide a non-contactingtemperature sensing device where fast response and accuracy are needed,such as for automotive applications, which reduces the impact ofemissivity dependence on the resulting device and which incorporatesmicro-bolometric detectors as the temperature sensing means.

[0012] In accordance with the invention, this object is achieved with anon-contacting temperature sensing device for automotive applicationscomprising:

[0013] a first infrared sensing means for detecting infrared radiationwithin a first wavelength band and for producing a first signalcorresponding to the detected infrared radiation of said firstwavelength band;

[0014] a second infrared sensing means for detecting infrared radiationwithin a second wavelength band and for producing a second signalcorresponding to the detected infrared radiation of said firstwavelength band;

[0015] signal processing means for obtaining the ratio of the first andsecond signals in order to provide a third signal, the third signalbeing emissivity independent and related to temperature.

[0016] Preferably, the device according to the invention, provides anon-contacting temperature sensing device incorporating micro-bolometricdetectors as the suitable infrared sensing means in detecting roadconditions for automotive applications. Further, the first and secondinfrared sensing means each include an active infrared sensing elementand a temperature drift compensating element. A current bias is appliedto the active infrared sensing element as well as to the temperaturedrift compensating element, which is identical in structure with theactive infrared sensing element, and the voltage outputs of these twoelements pass through a differential amplifier. The fluctuation in thesubstrate temperature or the ambient temperature affects the activesensing element and the compensating element in the same way, thus it iscancelled out. Instead of using one spectral band of the infraredradiation, as in the prior art, two spectral bands are used resulting ina first and second signal. The ratio signals are emissivity independent,so that the device of the present invention provides more accuratetemperature of the first and second measurements. The need to compensatefor window contamination is also eliminated by this two band approach

[0017] The filtering elements for the two bands could be multi-layerthin film filters either coated on flat windows or on diffractivemicro-lenses. The use of diffractive micro-lenses further reduces thesize of the device, and eliminates the need for a separate optical lens.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention and its advantages will be more easilyunderstood after reading the following non-restrictive description of apreferred embodiment thereof, made with reference to the followingdrawings in which:

[0019]FIG. 1 is a perspective view of a micro-bolometer as known in theprior art;

[0020]FIG. 2 is a schematic representation of a first and secondinfrared sensing means for use with the device of the invention;

[0021]FIG. 3 is a circuit diagram of the interconnection of the firstand second infrared sensing means, including associated signalprocessing means, for DC bias;

[0022]FIG. 4 is a circuit diagram of the interconnection of the firstand second infrared sensing means, including associated signalprocessing means, for pulsed bias;

[0023]FIG. 5a and 5 b are top plan and cross-sectional viewsrespectively of a casing according to a preferred embodiment of theinvention;

[0024]FIG. 6a is a cross-sectional view of a device according apreferred embodiment of the invention; and

[0025]FIG. 6b is a cross-sectional view of a device according to anotherpreferred embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0026] The invention concerns a non-contacting temperature sensingdevice 10, particularly used for automotive applications, although itwill be apparent to those skilled in the art that other applications canbe contemplated for the non-contacting temperature sensing device 10 ofthe invention.

[0027] The device 10 includes a first infrared sensing means 20 a fordetecting radiation within a first wavelength band and for producing afirst signal corresponding to the detected infrared radiation of thefirst wavelength band. The device 10 also includes a second infraredsensing means 20 b for detecting radiation within a second wavelengthband which is different from the first band and for producing a secondsignal corresponding to the detected infrared radiation of the secondwavelength band. The devices further includes signal processing means 30for obtaining the ratio of the first and second signals in order toprovide a third signal which is emissivity independent and proportionalto temperature. The device 10, to that effect, may also include atemperature indicating means 100 for indicating the measured temperaturecorresponding to the detected infrared radiation. As mentioned in thedescription of the prior art, the first and second infrared sensingmeans 20 a, 20 b are housed in a casing 70 (shown in FIGS. 6a and 6 b)and aimed at a road surface (however, in the case where the deviceaccording to the invention is used for other purposes, it should beunderstood that the first and second infrared sensing means 20 a, 20 bcan be aimed at any other object).

[0028] In accordance with the invention, the first and second infraredsensing means 20 a, 20 b preferably each comprise an active infrareddetecting element 21 a, 21 b providing an active signal and atemperature drift compensating element 22 a, 22 b providing an inactivesignal. The difference between the active and inactive signals isamplified, resulting in the first and second signals respectively.Preferably, the active infrared detecting elements 21 a, 21 b and thetemperature drift-compensating elements 22 a, 22 b are all identicalmicro-bolometers such as the one illustrated on FIG. 1.

[0029] Referring now to FIGS. 2 to 4, two preferred embodiments of theinvention are shown.

[0030] As shown in FIG. 2, four identical micro-bolometers 21 a, 21 b,22 a, 22 b are fabricated on substrate 25 as shown. Preferably, thesignal processing means 30 are also integrated on the substrate 25. Thebolometers 21 a, 21 b, 22 a, 22 b can have the same structure asbolometer 40 shown in FIG. 1 or any suitable micro-bolometer structures.As mentioned above, first and second micro-bolometers 21 a and 21 b areactive detecting elements, which are aligned with wavelength selectingmeans 51 a and 51 b, respectively. The wavelength selecting means willbe further detailed hereinafter. The purpose of wavelength selectingmeans, which are essentially band pass fitters with different bands, isthat the fluctuation in the substrate temperature or the ambienttemperature affects the active sensing element and the compensatingelement in the same way, thus it is cancelled out. Instead of using onespectral band of the infrared radiation, as in the prior art, twospectral bands are used resulting in a first and second signal. Theratio signals are emissivity independent, so that the device of thepresent invention provides more accurate temperature of the first andsecond measurements. The need to compensate for window contamination isalso eliminated by this two band approach.

[0031] Therefore, detector 21 a sees infrared radiation from wavelengthband 1 and detector 21 b sees radiation from band 2. Bolometers 22 a and22 b are temperature drift compensating elements, which are aligned withan opaque cover 52, so that they are not exposed to the infraredradiation from the radiating surface viz. the ground surface or roadway.The active detecting element 21 a and 21 b and the temperature driftcompensating element 22 a and 22 b could be a single largemicro-bolometer, or multiple small micro-bolometers connected in series.In the preferred embodiment, they are single micro-bolometric detectorsof the size of 150×150 μm. VO₂ film of temperature coefficient ofresistance (TCR) of −4% is sandwiched between a top and bottom siliconnitride layers which are the insulating dielectric layers. The nominalresistance of one micro-bolometer is 90 kΩ.

[0032] Since micro-bolometers are active elements, the device mustinclude power means. In this present case, reference will now be made tobiasing schemes and operation conditions. Two types of biasing schemesand operation conditions are preferred, i.e. continuous DC bias orpulsed bias, which are appropriate for different conditions, although itis possible that other biasing schemes are equally applicable to thepresent invention.

[0033] In vacuum operation, i.e. if the casing is under a vacuum, heattransfer through convection is greatly reduced. This results in bettersensitivity but also increases bolometer self-heating. In this case,pulsed bias is desirable since the pulse height and width can beadjusted so that the detectors can have maximum responsivity withoutinducing self-heating.

[0034] On the other hand, in normal air operation, the extent ofself-heating is reduced. DC bias thus becomes more effective becausesmall noise bandwidth can be achieved through passive filtering, andmore frequent sampling is possible. The device 10 of the invention canalternatively be sealed in a package filled with a low thermalconductivity inert gas to increase thermal insulation and reducemoisture build-up. This inert gas can be Xe, for example, which has fourtimes less thermal conductivity than normal air. In this case, theselection of biasing schemes should take in account many interrelatedfactors including the choice of inert gas, the pressure in the packageand the bolometer resistance. Current bias or voltage bias is equallypossible depending on specific bolometer and read-out designs.

[0035]FIG. 3 shows one embodiment of the first and second infraredsensing means and the signal processing means 30 for continuous DC bias.As mentioned above, infrared sensing means 20 a and 20 b are wavelengthfor band 1 and band 2 respectively. The infrared sensing means 20 a and20 b comprises the active sensing micro-bolometer 21 a and 21 b, and thetemperature drift compensating micro-bolometer 22 a and 22 b. Anintegrated low noise current source 25 provides current bias for allbolometric detectors. The infrared sensing means 20 a and 20 b can alsoinclude on-chip or off-chip capacitors (not shown) for passive filteringto reduce the noise bandwidth. The voltage output of micro-bolometers 21a and 21 b at nodes 26 a and 26 b and the voltage output ofmicro-bolometers 22 a and 22 b at nodes 27 a and 27 b are connected tothe inputs of a differential amplifiers 31 a and 31 b.

[0036] Under dark condition, all of the bolometers i.e. the activesensing bolometers 21 a and 21 b and temperature drift compensatingbolometers 22 a and 22 b are exposed to no infrared radiation.,Therefore, the voltage difference between nodes 26 a and 27 a and 26 band 27 b is ideally 0. When the active sensing bolometers 21 a and 21 bare exposed to the infrared radiation from the radiating surface, theirresistance will decrease due to the heating caused by the impinging ofthe infrared radiation. A small voltage difference proportional to thereceived infrared energy is induced across nodes 26 a and 27 a and 26 band 27 b. The voltage difference across nodes 26 a and 27 a correspondsto the signal from band 1 and the voltage difference across nodes 26 band 27 b corresponds to the signal from band 2. The amplified signalsare then preferably sampled and averaged by the signal average means 32a and 32 b before being converted to a digital signal by the ANDconverting circuits 33 a and 33 b. The averaging of the multi-samples ofthe signals is that noise is reduced and sensitivity increased. Thedigitised signal from band 1 is divided by the signal from band 2 by thesignal ratio means 34. The emissivity factor is cancelled out by thisdivision since both active micro-bolometers 21 a and 21 b are equallyaffected by a change in emissivity. Thus the resultant ratio varies onlyin proportion to changes in surface temperature and can be correlatedwith specific temperatures of the radiating surface independent of theemissivity. Accordingly, the output of signal ratio means 34 isconnected to a temperature indicating means 100 which may be calibratedto display numerical values of temperature corresponding to the receivedsignal ratio.

[0037]FIG. 4 shows another embodiment of the device under pulsed biasconditions. Active sensing bolometers 21 a and 21 b are connected withtheir counterparts temperature drift compensating bolometers 22 a and 22b in series to form one arm of the Wheatstone bridge. The other arm ofthe Wheatstone bridge is formed by connecting in series two stationaryresistors 23 a, 24 a and 23 b and 24 b, which are identical in structureand in resistance values. The magnitude of the value of resistance 23 a,24 a and 23 b, 24 b is set in conformity with the magnitude ofresistance of the micro-bolometers which, as mentioned previously, is inthe order of 90 kΩ. The stationary resistors 23 a, 24 a, 23 b, 24 b arefabricated on the same substrate as the electronic circuits andconnected with the micro-bolometers 21 a, 22 a, 21 b, 22 b to form theWheatstone bridges as shown in FIG. 4.

[0038] A pulsating voltage generated by pulse generating circuits 10 a,10 b is applied across nodes 28 a, 29 a and 28 b, 29 b to provide biasto the bolometers 21 a, 22 a, 21 b, 22 b. The first and second signalsare taken from the voltage difference between nodes 26 a′, 27 a′ and 26b′ and 27 b′. Under dark condition, the Wheatstone bridge is in perfectbalance. Therefore, the voltage difference between nodes 26 a′ and 27 a′and 26 b′ and 27 b′ is ideally 0. When the active sensing bolometers 21a and 21 b are exposed to the infrared radiation from a radiatingsurface, their resistance will decrease due to the heating caused by theimpinging of the infrared radiation. A small voltage differenceproportional to the received infrared energy is induced across nodes 26a′, 27 a′, 26 b′ and 27 b′. The resulting signals are then amplified bythe subsequent differential amplifiers 31 a′ and 31 b′ resulting in thefirst and second signals, respectively. The amplified signals frommultiple pulses are then averaged by the average means 32 a′, 32 b′before being converted to a digital signal by the A/D convertingcircuits 33 a′, 33 b′. As for DC bias, the averaging of signals frommultiple pulses for one temperature measurement further reduces noiseand increases sensitivity.

[0039] The performance of the bolometers in the preferred embodiment asshown in FIG. 2 and FIG. 3 is simulated. The estimated noise equivalenttemperature differential (NETD) in vacuum and in air is listed in Table1 for two selected wavelength bands, assuming f/2 optics, 720 pF passivefiltering, current bias of 15 μA, and 75-sample averaging in 0.1s. TABLEI NETD (K) Band 8.5-10.5 11.5-13.5 Scene Temp air Vacuum air Vacuum 233K0.621 0.036 0.648 0.038 300K 0.250 0.014 0.343 0.020

[0040] An important consideration is the selection of the bands for theactive infrared detector. The criteria for band selection are: the bandsshould be wide enough for sufficient irradiation energy to reach thebolometers; they should be close enough so that the emissivity will notvary a great deal; the ratio of signals between the two bands shouldvary monotonously with the surface temperatures and more preferably witha large slope.

[0041] In the preferred embodiment, the bands of 8.5-10.5 μm and11.5-13.5 μm wavelength are selected. Radiometric calculation based onPlanck's blackbody law showed that energy ratio between these two bandsincreases almost linearly with a slope of 0.005/°C. Other bandselections are also possible. The band selection is implemented by thewavelength selecting means.

[0042] In a preferred embodiment, the device 10 according to theinvention is provided with a window cover. The window cover 50 as shownin FIG. 2 consists of the band-pass filtering elements 51 a, 51 b andopaque cover 52, which can all be multi-layer thin-film coated. Filter51 a has high transmission in the wavelength range of 8.5-10.5 μm andlow transmission outside that range, while filter 51 b has hightransmission in the wavelength range of 11.5-13.5 μm and lowtransmission elsewhere. Opaque cover 52 has low transmission within thewhole infrared radiation spectrum. Filters 51 a, 51 b and cover 52 arepreferably made of the same material so that they will have similarbackground irradiation characteristics. They can be made from a singlepiece of Ge or ZnSe substrate, for example, differently coated indifferent areas. Alternatively, three pieces of the same material arethin film coated separately and then glued together to form a completewindow cover 50. They are then aligned with respective detectors on thesubstrate and assembled together in a metal casing 70 (shownschematically in FIG. 6). The opaque cover 52 can also be a part of themetal casing that emissive characteristics similar to those of filters51 a and 51 b.

[0043] A focusing lens 80 for focusing the infrared radiation onto themicro-bolometers increases the radiation flux reaching the detector, andthus improves the sensitivity of the device. A lens of f/2 can be used,for example. It can be made of Ge or polyethylene.

[0044] In another embodiment, the band-pass filtering elements 51 a and51 b can be diffractive microlenses. Diffractive microlenses are made ofconcentric surface relief structure etched in a suitable substrate, forexample, Ge or Si. FIGS. 5a and 5 b show top view and cross-sectionalviews respectively of a typical diffractive microlens 60. The reliefpattern 62 is etched into the substrate 61 by photolithography and dryetching. Since the microlenses are fabricated on one substrate with thesame precision as the micro-bolometers, they can be aligned perfectlywith corresponding active infrared sensing elements 21 a, 21 b. Thediffractive microlenses can be designed to serve as focusing elements aswell as filtering elements, because their focal point is wavelengthselective. Only infrared radiation within a certain wavelength range isfocused resulting in high power density within that wavelength band.Outside that band, the incident energy is not focused and so the powerdensity is low. Normally this low out-of-band power density should notbe of concern. Nevertheless, the multi-layer thin film coating can stillbe applied to the microlenses to further improve the filtering property.By using the microlenses, a separate focusing lens 80 is not required,resulting in a reduced device size and cost.

[0045]FIG. 6a and FIG. 6b schematically show the device 10 of theinvention, including a metal casing 70 with the window cover 50 and lens80. In FIG. 6a, the substrate 25, with all the fabricated infraredsensing means and processing electronics, is bonded to the metal casing70. The window cover 50 is housed in the metal casing, in front of thesubstrate. Window cover 50 consists of the filtering elements 51 a and51 b on a flat substrate. A focusing lens 80 of f/#2 is assembled infront of the window cover. It can be adjusted so that the infraredradiation from the radiating surface can be focused onto the activesensing elements. FIG. 6b shows the alternative embodiment where adiffractive microlenses as the focusing and/or filtering elements isused. The window cover 50′ consists of at least two microlenses 51 a′and 51 b′ corresponding to the selected two bands. The incident infraredradiation is filtered and focused by 51 a′ and 51 b′ onto respectiveactive sensing detectors on substrate 25.

[0046] The effects of the substrate temperature drift and backgroundirradiation are compensated by the temperature drift compensatingelements as mentioned above. Since the temperature drift compensatingelements 22 a and 22 b are identical to the active sensing elements 21 aand 21 b, the substrate temperature drift affects 21 a and 21 b the sameway as it affects 22 a and 22 b. Thus the relative resistance differencebetween 21 a, 22 a and 21 b and 22 b is not affected even if there issubstrate temperature drift. The background radiation is usually weaksince it is not focused onto the detectors. In a worst case scenariowhere this background radiation is significant with respect to thesignals, the active sensing elements 21 a and 21 b will receive as muchbackground radiation as the compensating elements 22 a and 22 b sincewindows 51 a, 51 b and 52 are either formed on one piece of material ormade of the same material. By the same token as in the substratetemperature drift situation, this effect will be cancelled out at theoutput of the differential amplifier 31 a, 31 b or 31 a′, 31 b′depending on the biasing scheme used.

[0047] The device 10, in order to function properly, must of course beproperly calibrated. Two-point calibration is necessary for the activedetectors in order to obtain the gain and offset value of individualdetectors. The calibration procedure can be described as follows. Thebolometer signal is measured at two different blackbody temperaturescorresponding to incident power of P₁ and P₂. From these twomeasurements, detector gain and offset α and β can be obtained andcompensation for the offset β can be applied to the signal. Table IIgives a mathematical representation of the situation. TABLE II P₁ P₂Uncorrected signal V₁ = αP₁ + β V₂ = αP₂ + β Corrected signal V₁ ^(c) =V₁ − β = αP₁ V₂ ^(c) = V₂ − β = αP₂

[0048] In general, the ratio of detector signals from two bands can bewritten, after correction, as$\frac{V_{B1}^{BB}}{V_{B2}^{C}} = {\frac{V_{B1} - \beta_{B1}}{V_{B2} - \beta_{B2}} = {\frac{\alpha_{B1}}{\alpha_{B2}}\frac{ɛ_{B1}}{ɛ_{B2}}\frac{P_{B1}^{BB}}{P_{B2}^{BB}}}}$

[0049] where ε is the road surface emissivity and P^(BB) is the poweremitted by a black body. If the bands are selected such thatε_(B1)=ε_(B2), then by knowing α and β,$\frac{P_{B1}^{BB}}{P_{B2}^{BB}} = {\frac{\alpha_{B2}}{\alpha_{B1}}\frac{V_{B1}^{C}}{V_{B2}^{C}}}$

[0050] through the calibration, the true temperature of the road can bededuced by comparing P_(B1) ^(BB)/P_(B2) ^(BB) with the ratio vs.temperature curve of an ideal blackbody.

[0051] An alternative approach to calibration is that the signal ratioV_(B1)/V_(B2) is calibrated using a blackbody source for all thetemperature points within the temperature range of interest (forexample, −40 to 100° C.), and a look-up table is generated. The truetemperature of the road can be derived by comparing the uncorrectedratio with the look-up table. This approach is straightforward, but thecalibration procedure requires more elaborate work.

[0052] It would be desirable that the non-contacting temperatemeasurement device be calibrated only once. However, it is difficult toguarantee that no drift would occur with time. The two-point calibrationapproach makes it easy for periodical re-calibration by a user. Anoffset update can be performed by using a surface at ambient temperatureor the surface of a block of ice as the reference temperature.

[0053] Theoretically, two active detectors and two reference detectorswould be enough to perform the temperature measurement. However, in caseof detector failure, it would be highly desirable that the completesystem does not break down. Redundancy of the IR detectors could beimplemented to avoid this problem. For example, five active and fivereferencing micro-bolometers could be used for each spectral band. Thatwould render twenty active detectors. The system could automaticallyswitch between detector pairs used for road temperature measurement if amicro-bolometer failure is detected. Factory calibration of these twentydetectors are as easy as for just 1. Or the twenty detector pairs couldbe read continuously and the valid temperature measurement be averaged(measurements from dead detectors would be discarded ). It isinteresting to note that this measure could be taken with almost noadditional fabrication cost since the die size would probably be thesame for 4 detectors or for 20 detectors.

[0054] One of the drawbacks of the prior art systems is that littleconsideration was given to window contamination. Due to typical outdoorconditions, the external window or lens will be subjected to dust,water, ice, mud, etc. The present invention using two wavelength bandsis much less sensitive to the contamination on the window than the priorart, where one-band method is used. On one hand, the windowcontamination attenuates the infrared radiation reaching themicro-bolometers the same way for both bands, i.e., the ratio of the twobands won't be affected. On the other hand, the window contaminationadds to the background radiation the same way for both the activesensing elements and the temperature drift compensating elements. Byusing the temperature drift compensating scheme, the fluctuation in thebackground radiation will not affect the measurement accuracy.

[0055] While the primary application for this device is for measuringthe temperature of the road surface from an automobile, it can also beused to measure the temperature of any part of the vehicle. For example,the thermal image of a rotating tire surface can be focused onto thisdevice and the tire temperature be measured. Due to the emissivityindependent nature of this invention, the emissivity of the tirematerial is not required.

[0056] From the foregoing it will be obvious to those skilled in the artthat many modifications, other than those already described anddiscussed herein, may be made without departing from the spirit of theinvention, as defined in the appended claims. It is to be understood,therefore, that all matter shown and described is to be interpreted asillustrative and not in a limiting sense.

1. A non-contacting temperature sensing device for use with a vehicle,comprising: a first infrared sensing means for detecting infraredradiation within a first wavelength band and for producing a firstsignal corresponding to said detected infrared radiation of said firstwavelength band; a second infrared sensing means for detecting infraredradiation within a second wavelength band and for producing a secondsignal corresponding to said detected infrared radiation of said firstwavelength band; signal processing means for obtaining the ratio of thefirst and second signal in order to provide a third signal, said thirdsignal being emissivity independent and proportional to a temperature.2. A non-contacting temperature sensing device according to claim 1 ,wherein: said first infrared sensing means comprises an active infrareddetecting element providing an active signal and a first temperaturedrift compensating element providing an inactive signal and said firstsignal corresponds to an amplified difference between said active signaland said inactive signal; said second infrared sensing means comprisesan active infrared detecting element providing an active signal and asecond temperature drift compensating element providing an inactivesignal and said second signal corresponds to an amplified differencebetween said active signal and said inactive signal, and said devicefurther includes power means for powering said first and second infraredsensing means.
 3. A non-contacting temperature sensing device accordingto claim 2 wherein: said device further includes a first wavelengthselecting means aligned with said first infrared sensing means and asecond wavelength selecting means aligned with said second infraredsensing means, and wherein said first and second wavelength selectingmeans allow passage of a first and second wavelength respectively, saidfirst and second wavelength being suitably close to one another in thespectrum to represent an emissivity ratio substantially equal to one. 4.A non-contacting temperature sensing device according to claim 3 ,wherein said first and second temperature drift compensating elementsare aligned with an infrared radiation opaque window.
 5. Anon-contacting temperature sensing device according to claim 4 , whereinsaid active infrared detecting elements of said first and secondinfrared sensing means are micro-bolometers.
 6. A non-contactingtemperature sensing device according to claim 5 , wherein saidtemperature drift compensating elements are micro-bolometers identicalto said micro-bolometers of said first and second infrared sensingmeans.
 7. A non-contacting temperature sensing device according to claim4 , wherein said first and second infrared sensing means and saidtemperature drift compensating elements each comprise a plurality ofidentical micro-bolometers connected in series, respectively.
 8. Anon-contacting temperature sensing device according to claim 3 , whereinsaid first and second wavelength selecting means are multi-layer thindielectric film band-pass filters.
 9. A non-contacting temperaturesensing device according to claim 3 , wherein said first and secondwavelength selecting means are diffractive microlenses.
 10. Anon-contacting temperature sensing device according to claim 4 , whereinsaid opaque window (52) has an emissive characteristic substantiallyequal to an emissive characteristic of said first and second wavelengthselecting means respectively.
 11. A non-contacting temperature sensingdevice according to claim 2 , wherein said power means include biasgenerating circuitry integrated with said infrared sensing means, andwherein said bias generating circuitry can generate continuous bias orpulsed bias.
 12. A non-contacting temperature sensing device accordingto claim 1 , wherein said first and second infrared sensing means arehoused in a casing and can operate in vacuum, in air or in an inert gasenvironment.
 13. A non-contacting temperature sensing device accordingto claim 1 , wherein said signal processing means comprises signalamplifier means, signal averaging means, analog to digital conversionmeans and signal ratio means, said signal processing means beingintegrated on a same chip as said first and second infrared sensingmeans.
 14. A non-contacting temperature sensing device according toclaim 1 , wherein said device includes a temperature indicating meansoperatively connected to said signal ratio means for receiving the ratiooutput signal therefrom and displaying a corresponding temperature of aradiating object.